Re: [PATCH] KVM: arm64: Add capability for unconditional WFx passthrough

[Colton Lewis]

Oliver Upton <oliver.upton@xxxxxxxxx> writes:

> Hi Colton,

> On Mon, Jan 29, 2024 at 09:39:17PM +0000, Colton Lewis wrote:
> > Add KVM_CAP_ARM_WFX_PASSTHROUGH capability to always allow WFE/WFI
> > instructions to run without trapping. Current behavior is to only
> > allow this if the vcpu is the only task running. This commit keeps the
> > old behavior when the capability is not set.

> > This allows userspace to set deterministic behavior and increase
> > efficiency for platforms with direct interrupt injection support.

> Marc and I actually had an offlist conversation (shame on us!) about
> this very topic since there are users asking for the _opposite_ of this
> patch (unconditionally trap) [*].

> I had originally wanted something like this, but Marc made the very good
> point that (1) the behavior of WFx traps is in no way user-visible and
> (2) it is entirely an IMP DEF behavior. The architecture only requires
> the traps be effective if the instruction does not complete in finite
> time.

> We need to think of an interface that doesn't depend on
> implementation-specific behavior, such as a control based on runqueue
> depth.

Good to know. I'll be thinking about that.


> [*]  
> https://lore.kernel.org/kvmarm/a481ef04-ddd2-dfc1-41b1-d2ec45c6a3b5@xxxxxxxxxx/

> --
> Thanks,
> Oliver